1. Field of the Invention
The present invention relates to a information terminal apparatus having a variable directional antenna, which is mounted to an information terminal unit such as a personal computer, a PDA (Personal Digital Assistant) and a cellular phone, in a system for serving data communications between a base station and an information terminal unit in accordance with a radio communication technology such as, for example, W-CDMA (Wideband Code Division Multiple Access) or wireless LAN, and a controlling method of the variable directional antenna.
2. Description of the Related Art
For a mobile communication terminal unit, the direction of the base station, i.e., the communication partner is not constant and changes depending on the location of the terminal unit, and thus it is believed that an antenna is preferably omnidirectinal as much as possible. For example, a monopole antenna, a helical antenna, an inverted-F internal antenna or the like is used for a cellular phone.
These antennas focus on reducing a size, improving efficiencies (cutting down losses), benefiting from the diversity effect and the like, but they are not intended to achieve high gain directivity. Some of the antennas are provided as external antennas for mounting on a body of a vehicle. They also focus on improving the antenna efficiencies and designed to obtain the maximum omnidirectivity in a plane.
However, it has been recently contemplated to introduce the HSDPA (High Speed Downlink Packet Access) system to W-CDMA. This is provided for performing CDMA (Code Division Multiple Access) data transmissions in a packet mode. As the solutions to the distance problem of CDMA, HSDPA takes approaches such as changing a spread or changing a modulation rather than controlling the power.
That is, a lower spread and/or a faster modulation are assigned to a channel with a low propagation loss (usually, a near user) to provide a high-speed data transmission. On the other hand, a higher spread and slower modulation are assigned to a channel with a high propagation loss (usually, a remote user) to lower the data transmission rate.
The reason for carrying out such operations is that the high speed data transmission with a low spread occupies a lot of CDMA channels, and thus increasing the transmission power results in interference with many other users accordingly. Therefore, the high-speed data transmission is provided only for the limited users who are located near the base station (i.e., users with a low propagation loss), while the remote users (i.e., users with a high propagation loss) are assigned with the low speed data transmission.
As a natural result of introducing such a system, higher data transmissions are allowed to a user with a lower propagation loss, and thus it is advantageous to reduce the propagation loss as much as possible. So the user wishing high speed data transmissions need to take the following two measures: 1) to conduct communications at a place as near the base station as possible where the base station can bee seen directly; and 2) to use directional antenna with high gain.
Among them, the first measure impairs a primary merit of the mobile communications that “communications can be made anywhere”. Accordingly, it is most realistic to take the second measure.
As is mentioned above, for the communication system such as HSDPA that will be newly introduced, it is more advantageous to use the directional antenna with high gain, and therefore it is predicted that the needs for the directional antenna increase.
On the other hand, a sector antenna for portable terminal units, the directivity of which can be switched to four directions is proposed in “Proceedings of the 2000 IEICE (Institute of Electronics, Information and Communication Engineers) General Conference, B-1-74 and B-1-75” (hereinafter referred to as Reference 1).
This sector antenna for portable terminal units is formed by a radiator disposed at the center of a circular ground board; and four passive or elements and switching circuits which are symmetrically arranged about the radiator, wherein only one of the passive elements toward the radiation direction is served as a director and the other three elements as reflectors, whereby the antenna directivity are made switchable to four directions.
Further, as a means for switching applications of the passive element between a director and a reflector, there are used an inductor for operating the element as a reflector by equivalently extending the electrical length of the passive element and a switching circuit comprising a PIN diode, FET, or the like for switching between presence and absence of the inductor.
In accordance with the sector antenna for portable terminal units disclosed in Reference 1, an antenna that is able to switch its directivity in the four directions can be implemented. However, such a system in which a passive element is simply switched to either a director or a reflector as mentioned above suffers the problems that it is difficult to allow switching the directivity in four or more multiple directions, and a sufficient gain can not be obtained because the directional characteristics exhibit a relatively wide directivity.
Consequently, a high gain directional antenna, which is switchable to more directions, becomes necessary. As an example of such antennas, it is considered to use an antenna described in “Design of Electronically Steerable Passive Array Radiator (ESPAR) Antennas” 2000 IEEE, AP-S International Symposium, PP. 922-925, July 2000 (hereinafter referred to as Reference 2), or described in Japanese Unexamined Patent Application (KOKAI) No. 2001-24431 (hereinafter referred to as Reference 3).
These antennas described in the above-cited documents References 2 and 3 also comprise a plurality of sub antenna elements symmetrically arranged on a circle about a main antenna element which is the only element being fed. A radio wave is radiated from the main antenna element, and then the respective sub antenna elements reflect the wave after appropriately shifting the phase of each sub antenna element. By adjusting the phase shift amounts of the sub antenna elements so that the wave fronts of the waves are aligned in a certain direction, it is possible to provide (for example, when 7 elements are used) a directivity with a gain of 8 dBi or more in a desired direction.
A variable phase shifter circuit for determining the phase shift amount can be made using a variable reactance element such as a varactor diode. According to the above-cited references, the directivity can be changed continuously, and further a scheme for determining each reactance value for providing a particular directivity is established.
A variable directional antenna with high gain can be achieved by using the antenna elements as mentioned above. Here, the problem associated therewith is how to control the respective sub antenna elements. It is certainly possible to determine the variable reactance values by using a certain algorithm. However, it is assumed that if a CPU incorporated in the information terminal unit directly calculates and supplies these values to the respective variable phase shifter circuit, the signal transmission between the antennas and the information terminal unit would become complex and the cost, size, and power consumption would be adversely affected.
In addition, it is believed that the variable directional antenna will be primarily used with the antenna being mounted outside the information terminal unit as an optional device. Therefore, it is required that connection between the antenna and the information terminal unit should be as simple as possible. Examples include connection using an only one single coaxial cable. In order to satisfy this requirement, a more simplified directivity control method and a simpler interface become necessary.